The invention relates to a printing or copying machine for single-color or multi-color simplex or duplex printing using a belt-type transfer element and a method for printing on recording media using a printing or copying machine of this type.
Printing or copying machines of the aforesaid type are disclosed, for example, in U.S. Pat. Nos. 4,477,176 and 4,537,493. These machines are electrophotographic devices with a photoconductor on which at least two charge images can be arranged one behind the other, the machines having a print-transfer station with associated single-sheet positioning device (turning device) which permits the electrophotographic printing device to be operated in two modes of operation, specifically in a first mode of operation in which the inked-in toner images which are arranged one behind the other on the charge image carrier are arranged one above the other or one next to the other on one side of the single sheet, and in a second mode of operation in which the sequence of toner images located on the charge image carrier is arranged on the front side and rear side of the single sheet. The fusing of the toner images on the single sheets is effected with the aid of a roller fusing station using pressure and very high temperatures.
In the duplex printing mode of operation in which a toner image is arranged on each side of the single sheet, the fusing of the front and rear side takes place simultaneously. For this purpose, the single sheet which is printed on on one side must be turned and printed on again on the other side, and subsequently it is conveyed in a contactless fashion to the roller fusing station, for example by means of an air cushion.
This requires a very high mechanical outlay if a high operational reliability is to be ensured for a wide range of printing materials.
Furthermore, U.S. Pat. No. 2,990,278 and Great Britain Patent 2,040,226 (corresponding to U.S. Pat. No. 4,453,820) disclose electrophotographic printing devices with which character-dependent charge images are produced on a photoconductor with the aid of an exposure device and are fed to a developing station. The developed charge image is then lifted off from the photoconductor mechanically by pressure with the aid of a belt-type transfer element and transferred to a recording carrier. In order to be able to fuse the toner image on the recording carrier, the toner image on the belt-type transfer element is heated with the aid of a heating device and the heated toner image is applied to the recording carrier by means of a roller arrangement using pressure and heat. After the transfer of the toner image to the recording carrier, the toner which is adhering is cleaned from the intermediate image-carrier in a cleaning station.
The transfer of the image information from the photoconductor to the belt-type transfer element is effected purely mechanically by contact pressure and rolling. The transfer element is relatively good at seizing individual toner particles mechanically. Print images which consist of a plurality of layers of toner are, however, only transferred inadequately or with a very poor transfer efficiency.
The result of this is that, when the belt-type transfer element is used in this way, firstly virtually no background particles can be developed on the photoconductor and, secondly, development toner technologies which result in a single-layer print information development have to be used.
The technology necessary for this (conductive single-component magnetic toner, highly sensitive photoconductor, very high exposure powers), is not appropriate for high-performance printers for reasons of economy and due to the technical complexity.
The conventional proven technology of high-performance printers operating according to the electrophotographic principle consists in the so-called two-component development method, a method in which a mixture of ferromagnetic carrier particles and toner particles is used as the developer mixture. This development method, referred to as dry inking, is cost-effective and is used in general in conjunction with arsenic triselenide photoconductor drums or OPC photoconductor belts. However, since this technology results both in a clear background inking and also in a plurality of layers of toner in the developed print image, the purely mechanical transfer onto a belt-type transfer element with very high print quality requirements is not possible.